Complementary metal-oxide-semiconductor (CMOS) imagers are conventionally fabricated by manufacturing a so-called imager wafer comprising an array of semiconductor imager devices each having an optically sensitive region thereon, that are mutually separated by boundary areas in the form of so-called “streets” along which dicing or singulating of the semiconductor devices may be effected by cutting.
A wafer assembly containing an array of the semiconductor imager devices is formed by aligning an optic wafer and an imager wafer and then securing the optic wafer and the imager wafer together. The optic wafer may comprise a wafer of glass or other transparent material, or may comprise a wafer level lens array comprising a plurality of lenses in a carrier substrate. In either case, optically transmissive regions of the optic wafer corresponding to locations of semiconductor imager devices on an imager wafer are separated by streets corresponding to street locations on the imager wafer. The optic wafer and the imager wafer are secured together by bonding the imager wafer and optic wafer together along the complementary streets with an adhesive such as epoxy, for example. The adhesive provides a bond thickness sufficient to facilitate singulation of the wafer assembly, as hereinafter described. While singulation of the wafer assembly may be further enhanced by increasing the bond thickness of the adhesive, the planarity of the wafer assembly may be compromised by such an increased bond thickness. It would be desirable to provide an assembly and singulation technique to maintain or improve the planarity of the wafer assembly while accommodating the known limitations of conventional equipment used to singulate or dice the wafer assembly.
In order to preserve the quality and integrity of the wafer assembly and the imager packages singulated therefrom during singulation, precision cutting along the streets to determinant depths with rotating saw blades, for example, is required. Otherwise, dicing of a particular layer of the wafer assembly may not be completed when a saw blade undercuts the wafer assembly, while damage may occur when a saw blade overcuts through a layer of the wafer assembly and into another layer. While undercutting requires reworking of the wafer assembly and undesirably results in lost productivity in order to complete the dicing process, overcutting results in a loss of, or damage to, the imager packages being singulated and may also cause damage to the cutting edge of a saw blade, resulting in lost time, product yield and money.
A conventional method of dicing a wafer assembly 20 comprising an optic wafer and an imager wafer, as noted above, requires utilization of an adhesive layer 26 to interface with the cutting edge of the saw blade in order to avoid damage to the wafer assembly 20 as shown in FIG. 1 during the singulation process. A rotating saw blade 30, provided on its cutting edge with a material suitable for cutting a semiconductor material, such as silicon, of an imager wafer and having a suitably shaped cutting edge, is shown cutting through an imager wafer 22 along a street 28 of the wafer assembly 20 and into the adhesive layer 26. In order to make a complete cut through the imager wafer 22 without causing damage to the rotating saw blade 30, the rotating saw blade 30 must penetrate through the semiconductor material of the imager wafer 22 and into, but not through, the adhesive layer 26. The thickness of adhesive layer 26 is on the order of 80 microns, having a normal variance of about plus or minus 10 microns, which may lead to undesirable penetration of the saw blade 30 into the material of optic wafer 24 during the dicing operation due to tolerance buildup of the various superimposed layers (imager wafer 22, adhesive layer 26, optic wafer 24) within the wafer assembly 20 or lack of adequate cutting depth tolerance accuracy of the saw blade 30. Should the saw blade 30 contact the material of the optic wafer 24, the saw blade 30 will break and also cause damage to at least portions of the wafer assembly 20.
After cutting of the imager wafer 22 is completed, another rotating saw blade 32, having a cutting edge configured with a material suitable for cutting a material of an optic wafer such as a glass (silicon dioxide, borosilicate glass, phosphosilicate glass, borophosphosilicate glass, polyimides, photopolymers, etc.) is shown at the right-hand side of FIG. 2 cutting through the optic wafer 24 along a street 29 of the wafer assembly 20 and into the adhesive layer 26 to finish the singulation process along street 29, where a cut through imager wafer 22 has already been effected. In order to complete dicing along a given street without damage, the cut made by the saw blade 32 must penetrate through the optic wafer 24 and into the adhesive layer 26 without contacting the imager wafer 22. As an example of an undesirable result, the saw blade 32 is shown cutting the street 28 beyond an acceptable depth criterion through and into the imager wafer 22 as shown by reference numeral 34 at the left-hand side of FIG. 2, potentially damaging one or more of the singulated semiconductor imager devices 36. It would be desirable to provide a wafer assembly that is capable of singulation without being affected by nominal variance in the adhesive layer, tolerance buildup or saw blade tolerance variations.
Accordingly, there is an ongoing desire to improve planarity of a multi-layer wafer assembly, such as an imager wafer assembly, without compromising the singulation process. There is a further desire to provide a multi-layer wafer assembly that is unaffected during singulation by nominal variances in thickness of the various layers of the assembly. There is also a desire to provide an optic wafer that accommodates the tolerance variations of the wafer assembly layers and equipment tolerances in order to improve the quality and resultant yield of the singulation process.